Amphibole

minerals

inosilicate

actinolite

edenite

Amphiboles are double chain inosilicates, such as
actinolite ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2,
edenite NaCa2Mg5(Si7Al)O22(OH)2,
hornblende ☐Ca2(Fe2+4Al)(Si7Al)O22(OH)2,
pargasite NaCa2(Mg4Al)(Si6Al2)O22(OH)2 and
tremolite ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22 (OH)2.
In the discontinuous branch of the Bowen reaction series amphibole is intermediate between pyroxene (higher temperature) and biotite (lower temperature).
Environments:

Plutonic igneous environments

Amphibole is typical of plutonic igneous environments.
Amphibole is a common but not essential constituent of rhyolite and gneiss.
It also may be found in granite, quartzolite, gneiss and eclogite.
Amphibole is the characteristic mineral of the amphibolite facies; it is never found in the granulite facies.

Alteration

Ca-Fe amphibole and anorthite to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2 (JVW p363)

calcium amphibole, calcite and quartz to diopside- hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction. (DHZ 2A p272)

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction. (DHZ 2A p272)

calcium-iron amphibole and anorthite to garnet (grossular and almandine), clinozoisite and quartz
Ca2Fe3Si8O22(OH)2 + 6Ca(Al2Si2O8) ⇌ 4/3Ca3Al2(SiO4)3 + 5/3Fe3Al2(SiO4)3 + 2Ca2Al3[SiO7][SiO4]O(OH) + 5SiO2
(MM 48.206)

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above. (DHZ 2A p453)

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction. (DHZ 2A p453)

chlorite (clinochlore), iron-poor epidote and SiO2 to amphibole (tschermakite), anorthite and H2O
3Mg5Al(AlSi3O10)(OH)8 + 6Ca2(Al2Fe3+)[Si2O7][SiO4]O(OH) + 7SiO2 → 5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2 + 2Ca(Al2Si2O8) + 10H2O
This reaction occurs at a fairly high metamorphic grade. (DHZ 3 p154)

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies, and it is the reason why amphibole is never found in granulite facies rocks. (DHZ 2A p139)

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